The present invention relates to an assembly for use in an apparatus serving for deaeration of treatment agents applied to webs of paper and board, whereby the assembly makes it possible to adjust the capacity of the air separator for different volumetric flow rates.
The invention also relates to a method for controlling the operation of an air separator.
In coating a web of paper or board, a treatment agent such as water-based size or pigment mixture is applied to the surface of the web to be treated. Conventionally, the treatment agent is applied in an excess amount and the surface of the treated web is smoothed by means of a doctor. The excess amount is returned after straining back to the main circulation of the treatment agent stock. Additionally, applicators of a treatment agent frequently use a flushing circulation or the like, wherefrom also a certain amount of the treatment agent is returned after straining back to the main circulation. Particularly during application and doctoring, air may have access into the treatment agent stock, whereupon the entrained air and bubbles may cause profile defects and even uncoated spots on the layer of applied treatment agent. Entrained air causes the greatest problems in coating with a pigment-containing coating mix and particularly problematic are such processes that involve high shear rates of the coating mix. The problem appears more accentuated with certain types of coating formulations that are prone to entrain large volume of air. Such formulations are talc-containing coating mixes, for instance. Accordingly, it is imperative to separate air as efficiently as possible from the web treatment agent before its pumping to the applicator apparatus.
In U.S. Pat. No. 4,170,457 is disclosed one kind of air separator comprising a cylindrical tank rotatable about a vertical axis. The coating liquid stock is pumped into the tank via its top and, under the effect of the centrifugal action, the coating liquid is driven onto the tank walls, whereby the air-laden portion of the coating stock remains in the radially central region of the tank and may thus be removed therefrom via the top of the tank. This apparatus is relatively complicated and needs a drive motor. As the apparatus contains moving parts, it needs scheduled maintenance and replacement of worn components. U.S. Pat. No. 4,390,351 describes an apparatus comprised of a spiraling tube or the like passage through which the liquid is forced to pass. The channel has specific compartments and spaces formed thereto, wherein the entrained air bubbles can be separated from the liquid circulating in the spiraling passage. While this apparatus features a simple construction, its efficiency is rather poor.
In FI Pat. No. 98,792 is disclosed a relatively efficient air separator apparatus. In this embodiment, the liquid or other mixture flowing therethrough is admitted tangentially into the upper region of a cylindrical separator tube, wherein the liquid is forced into a rapid vorticous motion. Herein, the air-laden fraction of the liquid is forced into the radially central region of the cylindrical separator tube and can be removed centrally from the tube at the lower end thereof. The air-free fraction is similarly removed from the lower end of the tube, at the radially marginal region of the tube. To attain a good air separation efficiency in this kind of apparatus, a relatively high velocity is required from the inlet flow. For this purpose, the diametral dimensions of the cylindrical separator tube and its inlet/outlet nozzles need to be made sufficiently small to ensure such a rapid flow velocity at the available flow rate. Another vital reason for the use of a small-diameter cylindrical separator tube is that hereby a short transit distance of entrained air bubbles from the peripheral region of the tube to the center of the tube becomes short, whereby a faster and thus more efficient separation is attained. Hence, this kind of air separator is generally operated in parallel groups so that the combined capacity of all the air separators in the group is sufficient for handling the required volumetric flow rate. The air separators are grouped so that, e.g., their inlet nozzles are connected to an inlet manifold and the reject/accept fraction outlet nozzles are respectively connected to a reject discharge manifold and an accept outlet manifold. In this fashion, the air separator assembly can be readily dimensioned for a desired capacity while simultaneously assuring a sufficiently high flow velocity in the separator units that is needed for efficient air separation. An air separator assembly formed according to these design rules may comprise, e.g., 5 to 30 separator units connected to common manifolds.
The individual separator units can be isolated from the manifolds by manual cut-off valves or plugs. During a normal production run, the output flow rate of the pump that circulates the treatment agent in the machine circulation of an applicator apparatus is substantially constant within a minor deviation range depending on the running speed of the applicator section of the coating mix or size. By setting the capacity of the air separator assembly through shutting off a suitable number of the separator units by plugging or operation of the manual cut-off valves so as to comply with the actual output of the machine circulation pump during the run, it is generally possible to match the capacity of the air separator assembly fairly well with the requirements of the applicator apparatus over its normal range of web speeds. Hence, there is no need during a normal production run for adjusting the air separator capacity.
During production shut-down periods and otherwise when no coat or size application is performed, the pump output is decreased to a low level of about 20% known as the stand-by circulation rate. It is disadvantageous to cut off the circulation entirely, because restarting the machine circulation of the treatment agent subsequently interferes with the run-up of the entire production line, and additionally incurs the risk of plugged circulation by dried clumps of the treatment agent. With the decreased overall circulation rate, also the flow rate through each separator unit falls even down to a level so low as to compromise efficient air separation in any one of the separator units. Resultingly, the stand-by flow rate produced by the circulation pump allows admission of air into the machine circulation of the treatment agent. During the restart of application and the run-up of the circulation pump output to that required for application, it takes several minutes to reduce the entrained air content of the treatment agent down to the low level corresponding to the normal application run state. During this transient period of time, the paper qualities such as its coat evenness, for instance, fall short of preset specifications thus requiring dumping of the produced web into the broke pulper. In fast-running applicators this causes a substantial production loss and, obviously, a longer production shut-down.
The above-described problem can be avoided by way of replacing the manual valves of the inlet, accept and reject fraction manifolds by automatic valves, whereby a desired number of separator units can be shut off from the circulation in the stand-by state. As such a shut-off operation must be carried out rapidly, manual valves are clumsy to manipulate when the separator assembly is to be adjusted to the stand-by state and, vice versa, it is practically impossible to open a large number of manual valves at the run-up of the circulation pump for the higher output. Herein, it must be appreciated that as each separator unit is equipped with three manual valves and the assembly may comprise up to 30 separator units, at the instant of circulation pump output increase for full-speed operation an enormous task emerges to open all the valves momentarily. As to the use of automatically controlled valves, this is a disadvantageous alternative inasmuch a large number of automatically controlled valves incurs a higher cost and the fitting of automatic valves with actuators into the limited space between the nozzles and the manifolds requires a greater space between the separator units and outdistancing of the same from the framework of the air separator assembly, whereby inevitably also the external dimensions of the air separator assembly become larger. This is awkward and, hence, retrofitting an air separator to the treatment agent machine circulation of older applicator sections in particular is frequently complicated by the insufficient footprint of available installation space. Accordingly, it is desirable to achieve a method and assembly that could permit the adaptation of an air separator in a simple manner to the requirements of a reduced volumetric flow rate of circulating treatment agent.
It is an object of the present invention to provide an assembly offering flow-rate-based capacity adjustment in equipment serving for air separation from treatment agent mixtures used in application to a web of paper or board.
The goal of the invention is achieved by virtue of equipping at least the inlet manifold or accept manifold of an air separator with a valve, advantageously an automatically controlled valve, that cuts off flow in the manifold so as to permit passage of the treatment agent flow from the manifold only to certain ones of the air separator units of the assembly.
According to a preferred embodiment of the invention, the automatically controlled valves are fitted not only to the inlet manifold, but also at respective points of the accept and reject manifolds.
The invention offers significant benefits.
The invention makes it readily possible to adapt an air separator for two different separation capacities. The modifications required on an existing air separator are minimal and the overall cost of the air separator is increased only slightly. The modification is mechanically and structurally simple to carry out and, moreover, the external dimensions of the air separator can be retained almost unchanged as compared to those of an unmodified separator. Resultingly, the air separator improved according to the invention can be fitted in a small space, which is an advantage in revamping the treatment agent machine circulation of older applicator sections.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are intended solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.